J. Phys. Chem. C 2009, 113, 19871–19874
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Fabrication of Metal and Metal Oxide Sponges by Self-Bubbled Triton X-45 Hydrogel Templates Farid Khan† and Stephen Mann*,‡ Nanomaterials Laboratory, Department of Chemistry, Dr HariSingh Gour UniVersity, Sagar- 470003, India, and Centre for Organized Matter Chemistry, School of Chemistry, UniVersity of Bristol BS 8 1 TS, U.K. ReceiVed: August 06, 2009; ReVised Manuscript ReceiVed: September 15, 2009
Triton X-45 was used as a sacrificial template to synthesize macroporous Ag, Au, or CuO monoliths via thermal treatment. Continuous networks of pores with sizes varied from 100 nm to 6 µm can be prepared through the use of additives such as dextran, silica nanoparticles, and the swelling agent, 1,3,5 trimethylbenzene (TMB), suggesting that the use of Triton X-45 gel offers a versatile route to the preparation of intact inorganic structures with controllable morphology. Introduction Porous materials have been used extensively in molecular catalysis, biosensor technology, photonic crystals, gas absorption, drug delivery, and nanoreactors.1-4 The fabrication of macroporous materials requires microstructural templates such as colloidal crystals, which have been chemically infiltrated with a range of precursors to produce metal oxide or metallic monoliths with ordered micrometre scale porosity.5 Recently, an alternative approach based on the use of a range of soft templates has been developed to prepare macroporous monoliths. For example, dextran hydrogels have been used to promote the thermally induced assembly of porous scaffolds of Ag, Au, CuO, and Fe oxide6 and porous networks of Au beads prepared using emulsion-templated polymers.7 Porous materials have also been synthesized in the presence of poly (ethyleneimine) hydrogels8 and open-framework TiO2 monoliths prepared using 1-buty-3 methyllimidazolium tetrafluoroborate9 or starch gels as templates.10 Porous Ag monoliths have also been synthesized using a silica hydrogel as a templating agent.11 In most cases, the monoliths exhibit disordered arrangements of macropores that are produced by thermally induced outgassing of the organic template in conjunction with localized deposition of the inorganic phase. In this paper, we report the use of gels of the surfactant Triton X-45 for the preparation of porous networks of Ag, Au, or CuO and investigate the influence that various additives, such as dextran, silica nanoparticles (Ludox), and the swelling agent 1,3,5 trimethylbenzene (TMB)12 have on the fabrication process.
Ag/Triton X-45/dextran (Mw ) 2 × 106, Fluka), Ag/Triton X-45/Ludox (As-40, colloidal silica, 40 wt % suspension in water, Sigma-Aldrich) and Ag/Triton X-45/TMB (Fluka) monoliths were prepared using the above protocol by adding 2.4 g of dextran (2.6 × 104 M, 34.87 wt %) in 4.5 g of water (65.22 wt %), 0.21 g of Ludox (4.03 × 102 M, 40 wt %), or 2.6 g of TMB (7.2 M, 86.5 wt %) separately to Ag/Triton X- 45 gel. The dextran- and TMB-containing gels were calcined at 600 °C with the above heating and cooling rates, whereas the Ag/ Triton X-45/Ludox gel was calcined at 800 °C for 2 h with heating and cooling rates of 2 °C/min. The calcined Ludox monolith was treated with 30% HF (Sigma, Aldrich) for 36 h, followed by repeated washing with water to remove silica from the monolith. Synthesis of Gold Sponges. Au/Triton X-45/dextran monoliths were prepared by dissolving 2 g of AuCl3 (Aldrich 99.99%) (21.67 M, 6.66 wt %) in 28 g of water (93.33 wt %) and adding 5.17 g of Triton X-45, followed by a paste of dextran prepared
Experimental Section Synthesis of Silver Sponges. Silver monoliths were prepared as follows. AgNO3 (2.4 g, 54.54 wt %, 7.06 M, BDH) was dissolved in 2 g of distilled water (45.45 wt %) and added to 2.4 g of Triton X-45 (Fluka) at room temperature. The mixture was stirred for 10 min to form a gel which gradually became dark in color. The resulting gel was aged for 48 h at room temperature and then calcined at 600 °C for 2 h at a heating rate of 1 °C/min, followed by cooling at a rate of 1 °C/min to room temperature in a Carbolite EFl furnace. * To whom correspondence should be addressed. E-mail: s.mann@ bris.ac.uk. † Dr HariSingh Gour University. ‡ University of Bristol.
Figure 1. SEM micrographs of Ag macroporous scaffolds prepared using Triton X-45 gel templates. Monoliths were prepared by calcination of (a) Ag/Triton X-45 gel, showing pores, 1-1.5 µm in size; scale bar ) 2 µm. (b) Ag/Triton X-45/dextran gel, showing increased pore sizes of 2-6 µm; scale bar ) 5 µm. (c) Ag/Triton X-45/Ludox gel after HF treatment, showing reduced pores sizes of 200-300 nm; scale bar ) 1 µm. (d) Ag/Triton X-45/TMB gel with wide range of pore sizes, from 1 to 3 µm; scale bar ) 5 µm.
10.1021/jp9076068 CCC: $40.75 2009 American Chemical Society Published on Web 10/13/2009
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Khan and Mann
Figure 2. XRD patterns of silver, gold, and copper oxide monoliths prepared using Triton X-45 gel templates. (a) Sample holder (poly(methyl methacrylate, PMMA), (b-e) Ag monoliths prepared from gels of (b) Ag/Triton X-45, (c) Ag/Triton X-45/dextran, (d) Ag/Triton X-45/Ludox (after HF treatment), and (e) Ag/Triton X-45/TMB. (f) CuO monolith prepared from Triton X-45/dextran gel. (g) Au monolith prepared from Triton X-45/dextran gel.(inset shows full range of scanned 2θ values).
by dissolving 16 g of dextran (0.0016 M, 76.2 wt %) in 5 g of water (23.8 wt %)), and stirring for 15 min. The resulting yellow gel was aged for 15 days to allow polysaccharide-mediated reduction of the Au(III) complex to metallic gold. The resulting gel was calcined at 800 °C at a heating rate 5 °C/min for 2 h, followed by cooling at the rate of 5 °C/min to room temperature. Synthesis of copper oxide sponges. In the case of CuO monoliths, 2 g of copper(II) nitrate pentahemihydrate (2.87 M, 40 wt %, Riedel-deHaen) was dissolved in 3 g of water (60.0 wt %) and 2 g of Triton X- 45 and a paste of dextran prepared by dissolving 2 g (0.0006M, 50 wt %) in 2 g of water (50 wt %), added. The resulting mixture was stirred using magnetic stirrer for 30 min at 50 °C to produce a blue-green gel that was aged for 4 days at room temperature. Then the gel was calcined at 600 °C for 2 h at heating and cooling rate of 5°/min. Characterization. Scanning electron microscopic analysis was performed using a JEOL 5600 microscope. The samples were placed on aluminum stubs coated with carbon films. CuO monoliths were coated with Pt and Pd. Powder X-ray diffraction (PXRD) patterns of monoliths were recorded on a Bruker D 8 diffractometer using Cu KR radiation. TGA profiles were obtained in air using a NETZSCH Simultaneous Thermal Analyzer (STA 409 EP) using a alumina reference crucible. Results and Discussion SEM and XRD Studies of Silver, Gold, and Copper Oxide Sponges. Calcination of Triton X-45 hydrogels containing Ag(I) ions produced an intact white monolith that was self-supporting and readily handled with tweezers. SEM studies revealed a continuous interconnected scaffold with a disordered network of pores ranging from 1 to 1.5 µm (Figure 1a). Highmagnification images indicate that the scaffold consisted of fused micro- and nanocrystallites, often 100-200 nm in size. Similar
microstructures were observed when dextran was added to the Ag/Triton X-45 gel except that the pore size was significantly increased to values of between 2 and 6 µm (Figure 1b). In contrast, much smaller pores in the size range of 200-300 nm were observed for monoliths prepared from Ag/Triton X-45 gels containing a Ludox sol after HF treatment (Figure 1c). The absence of silica nanoparticles in these monoliths was confirmed by EDAX analysis. Addition of TMB as a swelling agent to Ag/Triton X-45 gels produced monoliths with a marginal increase in pore size of 1-3 µm (Figure 1d). In each case, XRD studies of the Ag monoliths showed reflections at d spacings of 2.37, 2.06, 1.45, 1.23, 1.18, 1.02, 0.94, and 0.91 Å, which corresponded to the {111}, {200}, {220}, {311}, {222}, {400}, {333}, and {420} lattice planes of metallic silver with a facecentered cubic structure (Figure 2a-e). Similar experiments using Triton X-45/dextran gels containing AuCl3 or copper(II) nitrate produced intact macroporous sponge monoliths that were yellow or black, respectively (Figure 3). SEM images indicated that the pore sizes varied from 0.5 to 1 µm or 0.2 to 1.5 µm for the gold or copper oxide scaffolds, respectively. Gold particles of size 300 nm to 1 µm were observed in gold monoliths. XRD studies confirmed that these monoliths consisted of metallic gold with a face centered cubic structure, or CuO (tenorite) with a base-centered monoclinic unit cell (Figure 2f, g). TGA Study of Composite Gels. TGA of the Ag/Triton X-45 gel initially showed 18% wt. loss between 170-220 °C due to the removal of unbound Triton X-45, followed by 55% wt. loss between 220 and 400 °C due to the decomposition of silver nitrate and Triton X-45 (Figure 4a). Decomposition of AgNO3 into silver metal and release of O2 facilitated decomposition of the Triton X-45 matrix.6 A further 5% wt. loss was observed between 400-500 °C, as a result of oxidation of C to CO2,
Fabrication of Metal and Metal Oxide Sponges
Figure 3. SEM images of (a,b) gold monolith prepared by calcination of Au/Triton X-45/dextran showing pore sizes from 0.5 to 1.0 µm. (b) High-resolution image showing fused Au particles, 300 nm to 1 µm in size. (c, d) CuO monolith with pore sizes of 0.2 to 1.5 µm prepared by calcination of a CuO/Triton X-45/dextran gel.
resulting in a monolith containing a residue of 22% wt of silver. In comparison, TGA of the Ag/Triton X- 45/dextran gel showed a 9% wt. loss from 20 to 155 °C due to the removal of water, and a 23% wt. loss between 155-175 °C associated with
J. Phys. Chem. C, Vol. 113, No. 46, 2009 19873 removal of excess Triton X-45 (Figure 4b). Dextran, AgNO3 and Triton X-45 were removed at 175-305 °C (44% wt. loss), followed by the removal of C between 305 and 400 °C (6% wt loss), leaving 18% wt of Ag left in the macroporous monolith. In the case of the Ag/ Triton X-45/silica Ludox gel prepared without HF treatment, 7% and 16% wt. losses were observed between 20 and 190 and 190 and 200 °C due to removal of water and Triton-X-45, respectively, followed by decomposition of AgNO3 and the surfactant between 200 and 300 °C (40% wt loss), and removal of C from the monolith (6% wt loss) at 300-400 °C (Figure 4c). The final Ag/silica monolith constituted 31% of the initial mass. Analogous TGA studies for gels of Ag/Triton X-45 prepared in the presence of the swelling agent, TMB gave weight losses of 5, 15, 50 or 5% for removal/ decomposition of water (20-175 °C), TMB (175-200 °C), AgNO3 and Triton X-45 (200-360 °C), and C (360-450 °C), respectively, leaving 25% wt as the Ag monolith (Figure 4d). Control studies with a Ag/dextran gel indicated that reduction of AgNO3 to metallic Ag and decomposition of dextran occurred synergistically between 167 and 175 °C in the absence of Triton X-45 (Figure 4e). TGA profiles for CuO/Triton X-45/dextran gels showed a 2% wt. loss up to 114 °C due to the removal of water, followed by 25% wt loss at 114-150 °C associated with decomposition of copper(II) nitrate into CuO (Figure 4f). The concomitant release of O2 facilitated oxidative decomposition of the Triton X-45/
Figure 4. TGA curves for various gels; (a) Ag/Triton X-45, (b) Ag/Triton X-45/ dextran, (c) Ag/Triton X-45/Ludox, (d) Ag/Triton X-45/TMB, (e) Ag/dextran, (f) CuO/Triton X-45/dextran, and (g) Au/Triton X-45/dextran.
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dextran gel between 150 and 330 °C with considerable weight loss (60%), followed by removal of carbon (3% wt loss, 330-450 °C), and formation of the CuO monolith (residual wt, 12%). In case of the Au/Triton X-45/dextran gel, TGA indicated a complex thermal degradation process (Figure 4g). Water was removed below 167 °C (8% wt. loss), followed by decomposition of dextran, Triton X-45, and AuCl3 (to AuCl) between 167 and 330 °C (47% wt loss), and further decomposition of the surfactant and degradation of AuCl to Au and AuCl3 (minor phase, confirmed by XRD; 3AuCl ) AuCl3 + 2Au) between 330 and 550 °C (30% wt loss). At higher temperatures, carbon was removed (7% wt loss) to leave a gold monolith with a retained mass of 7% wt of the original gel. General Discussion. Our results indicate that macroporous monoliths of Ag, Au, or CuO can be readily prepared by thermal decomposition of Triton X-45 hydrogels containing appropriate reaction precursors. Imprinting of macropores in the metal or metal oxide monoliths is dependent on controlled outgassing of carbon dioxide bubbles into the viscous surfactant matrix, such that consolidation of the inorganic reaction products within interstices of the transient foam produces a continuous and interconnected scaffold rather than localized fragmentation. Significantly, we demonstrate that addition of dextran to the Triton X-45 hydrogels results in a significant increase in the pore size of the Ag monoliths, which we attribute to changes in the viscosity of the composite gels as well as their rate of decomposition. Moreover, addition of silica nanoparticles to the gels is shown to be a facile method for significantly reducing the pore size of these monoliths, suggesting that numerous other types of nanoparticles could be used to tailor the monolith architecture. Finally, we note that the control of pore size in monoliths prepared by in situ thermally induced gas bubble templating is dependent not only on the composition of the hydrogel matrix but also on the nature of the chemical reactions and crystal chemistry of the reaction products. For example, although gold monoliths with micrometer-sized pores could be readily obtained using AuCl3 as a precursor, thermal decomposition occurred at a significantly higher temperature than for silver nitrate due to the absence of released oxygen from decomposition of the inorganic salt. In other cases, for example, the thermal transformation of silver nitrate to silver nitrite followed by reduction to silver metal,6 release of oxygen not only facilitates oxidative degradation of dextran and Triton X-45, but also oxidation of C to CO2 at lower temperatures. On the other hand, release of oxygen from the thermal decomposition of copper(II) nitrate occurs at a lower temperature than for AgNO3, with the
Khan and Mann consequence that the rate of Triton X-45/dextran decomposition is accelerated and the concomitant pore sizes reduced in the resultant monolith. The pore size can be maintained, however, by incorporation of colloidal silica into the Ag/Triton X-45 gel and corresponding HF treatment of the monolith after calcination. Conclusions In conclusion, we report a simple method for exploiting hydrogels of Triton X-45 for the fabrication of intact macroporous monoliths silver, copper oxide, and gold using a thermally induced self-bubbling method. As Triton X-45 is a readily available surfactant, the described approach should offer a relatively low cost route to a wide range of inorganic macroporous monoliths with useful properties and applications. Acknowledgement. . We thank Ruchi Bilgainyan, DST Research Project Fellow, Department of Chemistry, Dr. Harisingh Gour University, Sagar, India for her contribution and Dr A. J. Patil and D. Walsh, School of Chemistry, University of Bristol, for help with various aspects of this work and Prof. G. Hutchings and his group, Cardiff Catalysis Institute, University of Cardiff for measuring the surface areas of monoliths. F.K. thanks the Commonwealth Scholarship and Fellowship Commission, London, for the award of Commonwealth Academic Staff Award Fellowship, and Dr. HariSingh Gour University, Sagar, India, for sanctioning his leave of duty. References and Notes (1) Jin, X. B.; Zhuang, L.; Lu, J. T. Electroanal. Chem. 2002, 519, 137. (2) Hieda, M.; Garcia, R.; Dixon, M.; Daniel, T.; Allara, D.; Chan, M. H. W. Appl. Phys. Lett. 2004, 84, 628. (3) Gao, S.; Wang, X.; Yang, J.; Zhou, L.; Peng, C.; Sun, D.; Li, M.; Zhang, H. Nanotechnology 2005, 16, 2530. (4) Hayes, J. R.; Nyce, G. W.; Kuntz, J. D.; Satcher, J. H.; Hamza, A. V. Nanotechnology 2007, 18, 1. (5) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531. (6) Walsh, D.; Arcelli, L.; Ikoma, T.; Tanaka, J.; Mann, S. Nat. Mater. 2003, 2, 386. (7) Zhang, H.; Hussain, I.; Brust, M.; Cooper, A. I. AdV. Mater. 2004, 16, 27. (8) Hua Jin, R.; Yuan, J. J. J. Mater. Chem. 2005, 15, 4513. (9) Yang, L.; Jung, L.; Meijia, W.; Zhiying, L.; Hongtao, L.; Ping, H.; Xiurong, Y.; Jinghong, L. Cryst. Growth Des. 2005, 5, 1643. (10) Iwasaki, M.; Davis, S. A.; Mann, S. J. Sol - Gel Sci. Tech. 2004, 32, 99. (11) Sisk, C. N.; Gill, S. K.; Weeks, L. J. H. Chem. Lett. 2006, 35, 814. (12) Ulgappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759.
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